At last year’s ML Family Workshop we presented an interface for let(rec) insertion – i.e. for generating (mutually recursive) definitions. We demonstrated its expressiveness and applications, but not its implementation, which relied on effects and compiler magic.

We now show how one can understand let insertion – and hence, implement it in plain OCaml. We give the first denotational semantics of let(rec)-insertion, which does not rely on any effects at all.